Photocytotoxic ternary copper(II) complexes of histamine Schiff base and pyridyl ligands

Ternary copper(II) complexes of salicylaldehyde-histamine Schiff base (HL) and pyridyl ligands, viz. [Cu(bpy)(L)](ClO 4 ) ( 1 ) and [Cu(dppz)(L)](ClO 4 ) ( 2 ), where bpy is 2,2 ′ -bipyridine (in 1 ) and dppz is dipyrido[3,2-a:2 ′ ,3 ′ -c]phenazine (in 2 ), were synthesized, characterized and their DNA binding, photo-activated DNA cleavage activity and photocytotoxicity studied. The 1:1 electrolytic one-electron paramagnetic complexes showed a d - d band near 670 nm in aqueous DMF (1:1 v/v). The crystal structure of complex 1 showed the metal in CuN 4 O distorted square-pyramidal geometry. Complex 2 intercalatively binds to calf-thymus (ct) DNA with a binding constant ( K b ) of ∼10 5 M −1 . It exhibited moderate chemical nuclease activity but excellent DNA photocleavage activity in red light of 647 nm forming •OH radicals. It showed remarkable photocytotoxicity in human cervical cancer cells (HeLa) giving IC 50 of 1.6 μ M in visible light (400-700 nm) with low dark toxicity. The photo-induced cell death is via generation of oxidative stress by reactive oxygen species. Graphical Abstract Schiff base copper(II) complex of dipyridophenazine shows remarkable photocytotoxic effect in HeLa cells in visible light (400-700 nm) with reduced dark toxicity via apoptotic pathway. The complex intercalatively binds to ct-DNA, exhibits moderate chemical nuclease activity but excellent DNA photocleavage activity in red light of 705 nm forming •OH radicals.